It sounds like you have a water problem. The drywall has probably had water get back there and now it's most likely gone. The only remedy at this point is to tear it back to the studs and redo your walls. If it's gotten to the studs; I don't have much hope for the curb and pan either. Tap something on them that won't crack the tile. They should sound solid if they're good. You won't hear a good sound if they're bad, more like just whatever tapping the tile as opposed to an echo sound if they were good.